1. Make sure a practise routine is set in place and followed.
Try for a consistent time Monday to Friday and one fun practise on the weekend! This gives you a day off as well. It might be a struggle at first to get used to the routine, but not only will this help with piano, but with a general outlook for tackling difficult projects in life!
2. Stay positive.
There are many ways to say "good job". Make sure to give tons of positive feedback about practise. Be sure to go around any negative comments (ie instead of "that's not C!", try using a question or a hint "remember where critter lives/remember what group of black keys the C is before?"). Be sure your child or significant other knows how much you love their music!